A new path to
solving a longstanding fusion challenge: Novel design
could help shed excess heat in next-generation fusion
power plants.
By David L. Chandler | MIT News Office
October 9, 2018 -- A class exercise at MIT, aided by industry
researchers, has led to an innovative solution to one of the longstanding
challenges facing the development of practical fusion power plants: how to get
rid of excess heat that would cause structural damage to the plant.
The new solution was made possible
by an innovative approach to compact fusion reactors, using high-temperature
superconducting magnets. This method formed the basis for a massive new
research launched this year at MIT and the creation of an independent startup
company to develop the concept. The new design, unlike that of typical fusion
plants, would make it possible to open the device’s internal chamber and
replace critical components; this capability is essential for the newly
proposed heat-draining mechanism.
The new approach is detailed in a
paper in the journal Fusion Engineering and Design, authored by Adam
Kuang, a graduate student from that class, along with 14 other MIT students,
engineers from Mitsubishi Electric Research Laboratories and Commonwealth
Fusion Systems, and Professor Dennis Whyte, director of MIT’s Plasma Science
and Fusion Center, who taught the class.
In essence, Whyte explains, the
shedding of heat from inside a fusion plant can be compared to the exhaust
system in a car. In the new design, the “exhaust pipe” is much longer and wider
than is possible in any of today’s fusion designs, making it much more
effective at shedding the unwanted heat. But the engineering needed to make
that possible required a great deal of complex analysis and the evaluation of
many dozens of possible design alternatives.
Taming fusion plasma
Fusion harnesses the reaction that
powers the sun itself, holding the promise of eventually producing clean,
abundant electricity using a fuel derived from seawater — deuterium, a heavy
form of hydrogen, and lithium — so the fuel supply is essentially limitless.
But decades of research toward such power-producing plants have still not led
to a device that produces as much power as it consumes, much less one that
actually produces a net energy output.
Earlier this year, however, MIT’s
proposal for a new kind of fusion plant — along with several other innovative
designs being explored by others — finally made the goal of practical fusion
power seem within reach. But several design challenges remain to be solved,
including an effective way of shedding the internal heat from the super-hot,
electrically charged material, called plasma, confined inside the device.
Most of the energy produced inside
a fusion reactor is emitted in the form of neutrons, which heat a material
surrounding the fusing plasma, called a blanket. In a power-producing plant,
that heated blanket would in turn be used to drive a generating turbine. But
about 20 percent of the energy is produced in the form of heat in the plasma
itself, which somehow must be dissipated to prevent it from melting the
materials that form the chamber.
No material is strong enough to
withstand the heat of the plasma inside a fusion device, which reaches
temperatures of millions of degrees, so the plasma is held in place by powerful
magnets that prevent it from ever coming into direct contact with the interior
walls of the donut-shaped fusion chamber. In typical fusion designs, a separate
set of magnets is used to create a sort of side chamber to drain off excess
heat, but these so-called divertors are insufficient for the high heat in the
new, compact plant.
One of the desirable features of
the ARC design is that it would produce power in a much smaller device than
would be required from a conventional reactor of the same output. But that
means more power confined in a smaller space, and thus more heat to get rid of.
“If we didn’t do anything about the
heat exhaust, the mechanism would tear itself apart,” says Kuang, who is the
lead author of the paper, describing the challenge the team addressed — and
ultimately solved.
Inside job
In conventional fusion reactor
designs, the secondary magnetic coils that create the divertor lie outside the
primary ones, because there is simply no way to put these coils inside the
solid primary coils. That means the secondary coils need to be large and
powerful, to make their fields penetrate the chamber, and as a result they are
not very precise in how they control the plasma shape.
But the new MIT-originated design,
known as ARC (for advanced, robust, and compact) features magnets built in
sections so they can be removed for service. This makes it possible to access
the entire interior and place the secondary magnets inside the main coils
instead of outside. With this new arrangement, “just by moving them closer [to
the plasma] they can be significantly reduced in size,” says Kuang.
In the one-semester graduate class
22.63 (Principles of Fusion Engineering), students were divided into teams to
address different aspects of the heat rejection challenge. Each team began by
doing a thorough literature search to see what concepts had already been tried,
then they brainstormed to come up with multiple concepts and gradually
eliminated those that didn’t pan out. Those that had promise were subjected to
detailed calculations and simulations, based, in part, on data from decades of
research on research fusion devices such as MIT’s Alcator C-Mod, which was
retired two years ago. C-Mod scientist Brian LaBombard also shared insights on
new kinds of divertors, and two engineers from Mitsubishi worked with the team
as well. Several of the students continued working on the project after the
class ended, ultimately leading to the solution described in this new paper.
The simulations demonstrated the effectiveness of the new design they settled
on.
“It was really exciting, what
we discovered,” Whyte says. The result is divertors that are longer and larger,
and that keep the plasma more precisely controlled. As a result, they can
handle the expected intense heat loads.
“You want to make the ‘exhaust
pipe’ as large as possible,” Whyte says, explaining that the placement of the
secondary magnets inside the primary ones makes that possible. “It’s really a
revolution for a power plant design,” he says. Not only do the high-temperature
superconductors used in the ARC design’s magnets enable a compact, high-powered
power plant, he says, “but they also provide a lot of options” for optimizing
the design in different ways — including, it turns out, this new divertor
design.
Going forward, now that the basic
concept has been developed, there is plenty of room for further development and
optimization, including the exact shape and placement of these secondary
magnets, the team says. The researchers are working on further developing the details
of the design.
“This is opening up new paths in
thinking about divertors and heat management in a fusion device,” Whyte says.
“All of the ARC work has been both
eye-opening and stimulating of new ways of looking at tokamak fusion reactors,”
says Bruce Lipschultz, a professor of physics at the University
of York, in the U.K., who was
not involved in this work. This latest paper, he says, “incorporates new ideas
in the field with the many other significant improvements in the tokamak
concept. … The ARC study of the extended leg divertor concept shows that the
application to a reactor is not impossible, as others have contended.”
Lipschultz adds that this is “very
high-quality research that shows a way forward for the tokamak reactor and
stimulates new research elsewhere.”
The team included MIT students
Norman Cao, Alexander Creely, Cody Dennett, Jake Hecla, Adam Kuang, Alex
Tinguely, Elizabeth Tolman, Hannah Hoffman, Maximillian Major, Juan Ruiz Ruiz,
and Brandon Sorbom, PSFC Research Scientists Daniel Brunner and Brian
LaBombard, Professor Dennis Whyte, and Mitsubishi Electric Research
Laboratories engineers Piyush Grover and Christopher Laughman. The work was
supported by MIT’s Department of Nuclear Science and Engineering, the
Department of Energy, the National Science Foundation, and Mitsubishi Electric
Research Laboratories.
